Select the topic sentence.

1what is amnesia? 2most of us think of a dramatic, total loss of memory brought on by a severe blow to the head. 3amnesia is, however, a very subtle, and sometimes sinister, illness. 4while amnesia can result from a head injury, overwork and worry can also lead to memory lapses. 5in some cases, diagnosis is difficult because these lapses are intermittent, rather than constant.
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the following key ideas correctly express the key sentence.

restate key idea: most people can learn to swim in ten short lessons.

restate key idea: the majority of water-accident victims require mouth-to-mouth breathing.

restate key idea: an empty, tightly closed gallon jug will support a tired swimmer.  

restate key idea: first aid is to prevent accidents as well as assist in rescue.

explanation: the key idea needs to have the same topic as the key sentence (drowning and safety).